We plan to continue work on the origin and role of "8S" DNA, the species of small double stranded DNA molecules which become highly labeled where mitochondria are incubated with DNA precursors. Hybridization studies will be done to elucidate any sequence homology between the "8S" DNA and mitochondrial high molecular weight DNA and between "8S" DNA and 7S D-loop DNA. Work is also in progress on the purification of the mitochondrial nicking-closing enzyme and on its putative role in DNA replication as well as on its mechanism of action. We are also seeking in mitochondria the "DNA gyrase" recently isolated by Gellert from E. coli. This enzyme catalyzes the ATP dependent conversion of relaxed to supercoiled DNA. BIBLIOGRAPHIC REFERENCE: Fairfield, F.R., Simpson, M.V., and Bauer, W.R. Purification and Properties of a DNA Nicking-closing Activity from Rat Liver Mitochondria. Fed. Proc., 35, 1595 (1976).